Thursday, February 18, 2010

How do I use the Grep command with regular expressions under Linux operating systems?

Linux comes with GNU grep, which supports extended regular expressions. GNU grep is the default on all Linux systems. The grep command is used to locate information stored anywhere on your server or workstation.

Regular Expressions
Regular Expressions is nothing but a pattern to match for each input line. A pattern is a sequence of characters. Following all are examples of pattern:

Wildcards
You can use the "." for a single character match. In this example match all 3 character word starting with "b" and ending in "t":

# grep '\' filename

Where,

* \< Match the empty string at the beginning of word * \> Match the empty string at the end of word.

Print all lines with exactly two characters:

# grep '^..$' filename
Display any lines starting with a dot and digit:

# grep '^\.[0-9]' filename
Escaping the dot

The following regex to find an IP address 192.168.1.254 will not work:

# grep '192.168.1.254' /etc/hosts
All three dots need to be escaped:

# grep '192\.168\.1\.254' /etc/hosts
The following example will only match an IP address:

# egrep '[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}' filename
The following will match word Linux or UNIX in any case:
# egrep -i '^(linux|unix)' filename
How Do I Search a Pattern Which Has a Leading - Symbol?

Searches for all lines matching '--test--' using -e option Without -e, grep would attempt to parse '--test--' as a list of options:

# grep -e '--test--' filename

How Do I do OR with grep?

Use the following syntax:

# grep 'word1|word2' filename
How Do I do AND with grep?

Use the following syntax to display all lines that contain both 'word1' and 'word2'

# grep 'word1' filenae | grep 'word2'
How Do I Test Sequence?

You can test how often a character must be repeated in sequence using the following syntax:

{N}
{N,}
{min,max}

Match a character "v" two times:

# egrep "v{2}" filename
The following will match both "col" and "cool":

# egrep 'co{1,2}l' filename
The following will match any row of at least three letters 'c'.

# egrep 'c{3,}' filename
The following example will match mobile number which is in the following format 91-1234567890 (i.e twodigit-tendigit)

# grep "[[:digit:]]\{2\}[ -]\?[[:digit:]]\{10\}" filename

How Do I Hightlight with grep?

Use the following syntax:
# grep --color regex filename
How Do I Show Only The Matches, Not The Lines?

. Matches any single character.
? The preceding item is optional and will be matched, at most, once.
* The preceding item will be matched zero or more times.
+ The preceding item will be matched one or more times.
{N} The preceding item is matched exactly N times.
{N,} The preceding item is matched N or more times.
{N,M} The preceding item is matched at least N times, but not more than M times.
- Represents the range if it's not first or last in a list or the ending point of a range in a list.
^ Matches the empty string at the beginning of a line; also represents the characters not in the range of a list.
$ Matches the empty string at the end of a line.
\b Matches the empty string at the edge of a word.
\B Matches the empty string provided it's not at the edge of a word.
\< Match the empty string at the beginning of word. \> Match the empty string at the end of word.

grep vs egrep
egrep is the same as grep -E. It interpret PATTERN as an extended regular expression. From the grep man page:

Traditional egrep did not support the { meta-character, and some egrep implementations support \{ instead, so portable scripts should avoid { in

# grep -E patterns and should use [{] to match a literal {.

GNU grep -E attempts to support traditional usage by assuming that { is not special if it would be the start of an invalid interval specification.
For example, the command grep -E '{1' searches for the two-character string {1 instead of reporting a syntax error in the regular expression.

POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.